In the field of bioscience and medicine, a systematic analysis of disease-related proteins is required for treatment and prevention of diseases. With the advancement in basic researches for drug discovery in molecular biology and genomics, the territory of drug discovery is changing rapidly and new methods are being developed for drug discovery as exemplified by the genomic drug discovery.
Also, in the field of bioscience and medicine including development of new drugs, identification of materials exhibiting physiological activities for specific diseases or under specific conditions is required. Since those biologically active substances are mostly proteins, elucidation of structures and functions of the proteins is of crucial importance.
Since the analysis of proteins is very complicated because of their various characteristics associated with molecular weight, isoelectric point (pI), hydrophilic or hydrophobic nature, or the like, it is needed to first separate the proteins and identify them based on mass spectrometry, bioinformatics, etc. Considering that the proteins related with diseases exist in relatively lower quantities than other proteins, high-performance protein separation techniques and low detection limits for the separated proteins are needed.
The electrospray ionization (ESI) technique was first introduced in 1984. Thermally unstable biochemical substances such as proteins, peptides and sugars are unsuited for structural analysis and characteristic study by gas chromatography-mass spectrometry (GC-MS).
For separation and fractionation of those thermally unstable biochemical substances, high-performance liquid chromatography (HPLC) is widely employed. For structural analysis of various biochemical substance separated by HPLC as well as qualitative and quantitative analysis, the biochemical substances dissolved in a solution are converted into charged ions by means of ESI and then injected into a mass spectrometer. Then, the mass spectrometer performs structural analysis through mass measurement of the injected ions using mass spectrometry (MS) spectrums and tandem spectrums (MS/MS spectrums).
The most frequently employed method for sample ionization by ESI is to inject the biochemical substances such as proteins or peptides eluted using a microsyringe pump or an HPLC pump capable of microflow rate control into a capillary emitter having an inner diameter of several to tens of micrometers (e.g. about 1 μm to about 20 μm). Then, by directly injecting heated nitrogen gas (e.g., to about 100° C. to about 300° C.) to the outlet port of the capillary emitter while applying a high voltage thereto, desolvation of the droplet formed by the electrospray is facilitated and ionization is enhanced.
However, since the nitrogen gas is injected directly to the capillary emitter, the capillary emitter may be shaken or the ions produced by the electrospray may be diffused. As a result, the movement of charged ions through the inlet port of the mass spectrometer may be affected.